Private branch exchanges (PBXs) using conventional wireline telecommunications technology are commonly found in corporate, campus and similar environments where communications internal to the organization or site are frequently required. Generally, a PBX or key sets system is closed, with service provided only to a given number of telephone units or approved users. Internal calls are then normally made on a fixed-fee or fixed-cost basis. The primary advantage of PBX systems is their ability to support internal communications without resort to the public switched telephone network. Among the additional advantages of PBX (closed) systems are their ability to support reduced-digit dialing, private telephonic messaging (phone mail) and private operator services.
With the advent of cellular telephone technology, efforts have been made to develop localized private wireless systems for use, for example, in a campus, small office, home office or similar "PBX" environment. While some such systems have been developed, and standards established for their implementation and use, these localized wireless systems are still subject to substantial limitations, primarily related to user capacity.
The current standards for the Advanced Mobile Phone System (AMPS) analog technology, in particular TIA IS-94 includes provisions for the implementation of analog PBX systems. However, the voice quality of analog systems is substantially inferior to digital systems. Further, the typical analog system employs Frequency Division Multiple Accessing (FDMA) which inefficiently uses the available bandwidth, which is at a premium. Specifically, in FDMA, channels are differentiated by frequency alone, with only one user supported per channel. Since the bandwidth available to a given provider is limited by government allocation, the number of users the provider can service at one time (i.e. the capacity) is proportionately limited.
Standards for implementation of local area (private) wireless service, such as PBX, have also been established for Time Division Multiple Access (TDMA) systems, such as in TIA IS-136. TDMA is a digital technology which improves on the analog FDMA technology, and in particular triples the capacity over FDMA. In a TDMA system, the bandwidth available to a provider is divided into channels by frequency, as in FDMA, and then the channels are divided time-wise into slots, with two slots per user (using current full rate vocoder techniques). Essentially, multiple users time-share the same frequency band. Currently, 6 slots per channel are available in this technology, and hence three users can be supported on one channel. Nevertheless, even though the TDMA technology generally provides a 3-fold capacity increase over the FDMA technology, that increased capacity is still insufficient to meet increasing user demand.
Code division multiple access (CDMA), as defined in TIA IS-95, is currently the technology which most efficiently uses the available bandwidth. Along with increased capacity, CDMA also requires less frequency planning and the voice quality is improved due to soft handoffs. In CDMA, all users receive the entire signal and then filter out their respective information based on coding. Typically, CDMA uses direct sequence spread spectrum transmissions to and from pseudo-orthogonal users. One CDMA channel is 1.25 MHz wide, which is equivalent to forty-two 30 KHz AMPS or TDMA channels. However, in CDMA today approximately twenty users can be supported per sector on a 1.25 MHz channel while in AMPS, only two users can be supported per sector for the same spectrum, since those forty-two channels must be distributed across twenty-one sectors. Hence, CDMA generally provides theoretically a ten-fold improvement in capacity over AMPS. Up to this point, however, it has not been possible to use CDMA technology to support local (private) wireless systems.
Private systems can be supported by the TDMA and FDMA technologies since is it possible to implement "frequency reuse" with TDMA or FDMA. For example, with typical frequency planning, a given geographic region may be divided into 7 areas or macrocells, with each macrocell in turn partitioned into 3 sectors. The available frequency spectrum is then allocated across the seven macrocell patterns, with three control channels and fifty-seven voice channels generally allocated to each macrocell. In turn, each macrocell is partitioned into three sectors with one control channel and nineteen voice channels allocated to each sector. For a localized or private system, a low power microcell is overlaid over a portion of a given sector. Interference between the microcell and the macrocell is then controlled by frequency scanning and identification of the unused frequency bands and/or slots which can be used for private/local wireless transmissions.
For the same reasons that CDMA allows for more capacity per bandwidth, CDMA will not economically support private wireless exchanges for a closed user group on the same RF channel as the macro system. Specifically, since all macrocells and sectors are on a single channel, it is not possible to "steal" a channel or slot for private use. Users are differentiated by the coding, and interference between cells is essentially controlled by controlling the power level between the base stations and the mobile units. The overlay of a microcell in such a system is highly impractical. Among other things, microcell transmissions can overpower macrocell transmissions to nearby mobile units. Similarly, passing mobile units, not part of the private system, can overwhelm the microcell base station to the detriment of the private user group. While it is possible to hand-off power control of mobile units roaming into the coverage area of a microcell to the microcell base station, the burdens on the private system may be dramatically increased. In particular, not only is control of all private users required, but also that of any mobile unit which randomly enters the coverage area, which would place an excessive burden on the microcell base station, depending on the number of mobile units transitioning the coverage area.
Another possible approach is to assign each private system a dedicated CDMA RF channel. This approach, however, would be substantially complex and costly to the service provider. First, frequency replanning would be required to accommodate each private channel vis-a-vis the existing public channels. Second, by dedicating a CDMA channel to private use, a corresponding CDMA channel is no longer available for public use throughout the coverage area. In other words, a dedicated CDMA channel for private use will render forty-two AMPS public channels unavailable. The problem only becomes compounded as more resources are transferred from public to private systems. Since service providers are generally concerned with optimizing revenue, the option of dedicating channels to closed groups in small coverage areas with limited revenue potential at the expense of potentially higher revenue public uses is unacceptable.
Other proposed approaches for implementing private CDMA systems in view of the interference problems have included establishing "guard zones" around each microcell and desensitization. In the guard zone approach, microcell to macrocell interference would be reduced by confining all the private mobile systems to a specified region around the microcell base station. This approach is not practical. First, confining the mobile stations into a specific area is difficult. Second, even though some natural attenuation of the transmitted signals can occur, a system of artificial attenuation to control signal levels within the microcell would still be required.
In the desensitization approach, the noise floor would be raised such that a soft handoff naturally occurs at the macrocell--microcell boundary. In other words, the sharpness of signal difference at the microcell/macrocell boundary is reduced by the higher noise floor such that the mobile station naturally transitions from one microcell/macrocell to the other. The same problem discussed above still results. Namely, the microcell base station now must control all mobile stations which cross into the microcell region, whether or not those mobile stations belong to the closed user group.
Thus, the need has arisen for circuits, systems and methods for implementing a private (local) wireless communications system while still maintaining the availability of CDMA technology. Such circuits, systems and methods should, among other things, allow establishment of a private wireless system in which interference between a macrocell, non-user mobile units, and the private system is eliminated or substantially reduced. In doing so, the burden of power control should not be unduly placed on the private system. Additionally, the convenience of the private mobile units should also be maintained.